U.S. patent number 9,494,453 [Application Number 14/398,374] was granted by the patent office on 2016-11-15 for ultrasonic sensor for high temperature and manufacturing method thereof.
This patent grant is currently assigned to SONIC CORPORATION, WOOJIN INC.. The grantee listed for this patent is SONIC CORPORATION, WOOJIN INC.. Invention is credited to Fumiyasu Ikarashi, Hyeon Kyu Joo, Hideo Kawaguchi, Ki Han Nam, Noriaki Saito, Minehiro Tonosaki.
United States Patent |
9,494,453 |
Nam , et al. |
November 15, 2016 |
Ultrasonic sensor for high temperature and manufacturing method
thereof
Abstract
Provided is an ultrasonic sensor which includes a piezoelectric
vibrator made of a lithium niobate (LN) single crystal and may be
used in a high temperature region by generating a high ultrasonic
wave output, and prevents cracks from being generated in the
crystal. A piezoelectric vibrator 1 of the present invention has a
surface (Y-axis 36.degree. cut surface) obtained by rotating a
surface orthogonal to a Y-axis of the LN crystal about an X-axis by
36.degree..+-.2.degree. as an output surface. The ultrasonic sensor
further includes a retarder 3 made of titanium and a bonding layer
2 for bonding one surface of the retarder 3 to the output surface.
The bonding layer 2 is made of silver and frit glass, and the frit
glass has a coefficient of linear expansion ranging from
5.times.10.sup.-6 K.sup.-1 to 15.times.10.sup.-6 K.sup.-1.
Inventors: |
Nam; Ki Han (Hwaseong-si,
KR), Joo; Hyeon Kyu (Hwaseong-si, KR),
Tonosaki; Minehiro (Tokyo, JP), Ikarashi;
Fumiyasu (Tokyo, JP), Kawaguchi; Hideo (Tokyo,
JP), Saito; Noriaki (Tokyo, JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
WOOJIN INC.
SONIC CORPORATION |
Hwaseong-si
Tokyo |
N/A
N/A |
KR
JP |
|
|
Assignee: |
WOOJIN INC. (Hwaseong-Si,
KR)
SONIC CORPORATION (Tokyo, JP)
|
Family
ID: |
51625719 |
Appl.
No.: |
14/398,374 |
Filed: |
March 25, 2014 |
PCT
Filed: |
March 25, 2014 |
PCT No.: |
PCT/KR2014/002488 |
371(c)(1),(2),(4) Date: |
October 31, 2014 |
PCT
Pub. No.: |
WO2014/157907 |
PCT
Pub. Date: |
October 02, 2014 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20160003654 A1 |
Jan 7, 2016 |
|
Foreign Application Priority Data
|
|
|
|
|
Mar 25, 2013 [KR] |
|
|
10-2013-0031672 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04R
17/00 (20130101); G01N 29/245 (20130101); G01F
1/662 (20130101); H01L 41/313 (20130101); G01F
1/66 (20130101); H01L 41/0815 (20130101); G01N
29/22 (20130101); G01N 29/228 (20130101) |
Current International
Class: |
G01F
1/66 (20060101); G01N 29/22 (20060101); H04R
17/00 (20060101); G01N 29/24 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
62006598 |
|
Jan 1987 |
|
JP |
|
04-029056 |
|
Jan 1992 |
|
JP |
|
07-046095 |
|
Feb 1995 |
|
JP |
|
10-339722 |
|
Dec 1998 |
|
JP |
|
11-054531 |
|
Feb 1999 |
|
JP |
|
2000-162004 |
|
Jun 2000 |
|
JP |
|
2005-064919 |
|
Mar 2005 |
|
JP |
|
2006-090804 |
|
Apr 2006 |
|
JP |
|
2008-1602092 |
|
Jul 2008 |
|
JP |
|
2008-256423 |
|
Oct 2008 |
|
JP |
|
10-2004-0089484 |
|
Oct 2004 |
|
KR |
|
10-2011-0079096 |
|
Jul 2011 |
|
KR |
|
Other References
English Translation of International Search Report from
PCT/KR2014/002488; dated May 26, 2014. cited by applicant .
R.Kazys, et al., "Research and development of radiation resistant
ultrasonic sensors for quasi-image forming systems in a liquid
lead-bismuth," ISSN 1392-2114 Ultragarsas (Ultrasound), vol. 62,
No. 3, pp. 7-15, 2007. cited by applicant .
Q. F. Zhou, et al., "Design and modeling of inversion layer
ultrasonic transducers using LiNbO3 single crystal," Ultrasonics,
vol. 44, Supplement, pp. e607-e611, 2006. cited by applicant .
Ikeda Dakuro, "Basis of Piezoelectric Material", OHM Co. 1984.
cited by applicant .
K. K. Wong edit, "Properties of Lithium Niobate," EMIS datareviews
series No. 28, Inspec, 2002. cited by applicant.
|
Primary Examiner: Colilla; Daniel J
Attorney, Agent or Firm: Baker & Hostetler LLP
Claims
The invention claimed is:
1. An ultrasonic sensor, comprising: a piezoelectric vibrator which
is made of lithium niobate and has a surface obtained by rotating a
surface orthogonal to a Y-axis of a lithium niobate crystal about
an X-axis by 36.degree..+-.2.degree. as an output surface; a
retarder made of titanium; and a bonding layer for bonding one
surface of the retarder to the output surface, wherein the bonding
layer is made of silver and frit glass, and the frit glass has a
coefficient of linear expansion ranging from 5.times.10.sup.-6
K.sup.-1 to 15.times.10.sup.-6 K.sup.-1.
2. The ultrasonic sensor of claim 1, wherein the opposite surface
of the retarder has a couplant layer containing silver.
3. The ultrasonic sensor of claim 2, wherein the couplant layer is
made of silver and frit glass, and the frit glass has a coefficient
of linear expansion ranging from 5.times.10.sup.-6 K.sup.-1 to
15.times.10.sup.-6 K.sup.-1.
4. The ultrasonic sensor of claim 1, wherein a mass ratio of the
silver to the frit glass in the bonding layer is in a range of
79:2.3 to 82:2.5.
5. The ultrasonic sensor of claim 1, wherein the retarder is made
of pure titanium inevitably including impurities.
6. The ultrasonic sensor of claim 1, wherein when a length of the
retarder is L, a sound velocity in the retarder is v, a use
frequency of the ultrasonic sensor is f, and the number of waves in
a burst wave driving the ultrasonic sensor is N, the relationship
of (L/v)>(N/f) is satisfied.
7. A manufacturing method of an ultrasonic sensor having lithium
niobate as a piezoelectric vibrator, comprising: forming the
piezoelectric vibrator which has a surface obtained by rotating a
surface orthogonal to a Y-axis of a lithium niobate crystal about
an X-axis by 36.degree..+-.220 as an output surface, and is
prepared by applying a silver paste to at least the output surface
and burning the same; applying the silver paste to one surface and
the other surface of a titanium retarder which are opposite to each
other; and contacting one surface of the retarder with the output
surface of the burned piezoelectric vibrator, and then performing
burning the same under an inert gas atmosphere at a predetermined
temperature or more, wherein the silver paste applied to the output
surface and the one surface of the retarder includes silver and
frit glass, and the frit glass has a coefficient of linear
expansion ranging from 5.times.10.sup.-6 K.sup.-1 to
15.times.10.sup.-6 K.sup.-1.
8. The method of claim 7, wherein a composition of the silver paste
applied to the other surface of the retarder is the same as that of
the silver paste applied to the one surface thereof.
9. The method of claim 7, wherein the silver paste includes an
organic binder, and a mass ratio of the silver to the frit glass in
the silver paste has a value ranging from 79:2.3 to 82:2.5.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a National Stage of International Patent
Application No. PCT/KR2014/002488, filed Mar. 25, 2014 and claims
priority to foreign application KR 10-2013-0031672, filed Mar. 25,
2013, the contents of which are incorporated herein by reference in
their entirety.
TECHNICAL FIELD
The present invention relates to an ultrasonic sensor, and more
particularly, to an ultrasonic sensor suitable for measuring a flow
rate of a high-temperature fluid and a manufacturing method
thereof.
BACKGROUND ART
An ultrasonic flowmeter emits an ultrasonic wave into a fluid,
receives the emitted ultrasonic wave to obtain a flow velocity, and
converts the obtained flow velocity into a flow rate of a fluid to
measure the flow rate (Non-Patent Document 1). An ultrasonic sensor
which is a piezoelectric vibrator is used to emit or receive the
ultrasonic wave. As a method of obtaining the flow velocity, there
is a method using a Doppler effect, and the like. However, a method
for measuring a transfer time difference, which uses ultrasonic
sensors disposed at an upstream side and a downstream side of a
pipe, respectively, and includes: obtaining a flow velocity based
on a difference between a propagation time of an ultrasonic wave
transmitted to the upstream side and a propagation time of an
ultrasonic wave transmitted to the downstream side; and calculating
the flow rate by the obtained flow velocity, has been widely
used.
The method for measuring the transfer time difference uses a gate
configured to measure time based on an ultrasonic wave transmitting
timing and an ultrasonic wave receiving timing between the
ultrasonic sensors disposed at the upstream and downstream sides of
the pipe, and a high speed counter to measure an ultrasonic
propagation time to the upstream side and an ultrasonic propagation
time to the downstream side. As a timing detecting method, there is
a zero-crossing method to measure a position at which the received
ultrasonic signal is zero-crossed.
Meanwhile, a correlation method obtains the propagation time to the
upstream side and the propagation time to the downstream side based
on an autocorrelation peak time of a transmission waveform and a
reception waveform.
The ultrasonic flowmeter has also been used in equipments such as a
boiler to measure a flow rate of a fluid under high temperature and
high pressure conditions. When an outlet temperature of the boiler
is about 100.degree. C., a sensor using piezoelectric zirconate
titanate (PZT) which is piezoelectric ceramic has been mainly used
in the related art. However, a Curie point of the PZT is about 150
to 250.degree. C. depending on a composition thereof, and a
piezoelectric constant thereof is remarkably reduced in the
vicinity of the Curie point. For this reason, in order to measure a
flow rate of a fluid in a region exceeding 200.degree. C., a sensor
using a piezoelectric material or a piezoelectric single crystal
material having a higher Curie point than the PZT has been used
(Non-Patent Document 2).
As the method of ultrasonic flow measurement in a high temperature
region, a method using a sensor of a conventional PZT based
material by cooling the sensor has been proposed. For example,
Patent Document 1 discloses a method for obtaining a flow rate of a
high temperature fluid according to a transfer time difference
principle by disposing a pipe through which the high temperature
fluid flows within a container filled with a low temperature liquid
and disposing ultrasonic sensors on pipe walls of the upstream and
downstream sides of the pipe so that the ultrasonic sensors are
cooled by the low temperature liquid. Further, Patent Document 2
discloses a configuration to prevent a temperature of the
piezoelectric vibrator from increasing due to heat from the high
temperature fluid by installing a sound transmission passage of a
quartz material between the piezoelectric vibrator and the high
temperature fluid at the time of measuring the flow rate of the
high temperature fluid.
Meanwhile, as a piezoelectric material, there is lithium niobate
(LiNbO.sub.3: hereinafter briefly referred to as an LN) which has a
much higher Curie point than the PZT and may withstand high
temperature conditions. General properties of the LN are described
in Non-Patent Document 5. The Curie point of the LN is about
1200.degree. C. FIG. 1(a) illustrates a crystal structure of the
LN. The LN has a crystal structure of a trigonal system and as
illustrated in FIG. 1, an X-axis, a Y-axis, and a Z-axis are
crystallographically defined. Further, as a lattice constant of the
LN, a=b=5.148 .ANG. and c=13.863 .ANG..
When the LN is used as the ultrasonic sensor of the ultrasonic
flowmeter, an ultrasonic wave having a short duration needs to be
generated as a burst wave and thus vibration needs to be dumped. In
order to dump the vibration, a metal piece (that is, dumper) is
attached to the LN vibrator. As the attachment position of the
metal piece, there are two cases, that is, a case in which the
metal piece is attached on the same surface as an output surface of
the ultrasonic wave in the ultrasonic sensor as described in Patent
Document 4, or the like, and a case in which the metal piece is
attached on a surface opposite to the output surface of the
ultrasonic wave as described in Patent Document 5, or the like.
Patent Document 4 discloses that an aluminum alloy lead material is
used for bonding the dumping portion to the piezoelectric vibrator,
and Patent Document 5 discloses that silver (Ag) is used as the
dumping portion for bonding the dumping portion to the
piezoelectric vibrator by eutectic bonding between thin films of
silver and gold (Au). Further, Patent Document 7 discloses that a
metal shoe which forms a temperature gradient while serving as the
dumping portion is bonded to the piezoelectric vibrator made of a
ferroelectric material having a high Curie point. In addition,
Patent Document 10 discloses that as a lead material for performing
the bonding to the LN piezoelectric vibrator, an Al--Si--Mg alloy
or a silver solder is used and as the silver solder, a material
containing 45% Ag, 16% Cu, 24% Cd and the remainder being Zn is
used. Further, Patent Document 10 discloses that when the LN
piezoelectric vibrator is bonded to a protective layer made of a
cermet insulating material, a thin film of Cu or Ni is formed on a
surface of the cermet insulating material by an ion plating, a
silver electrode is formed on the piezoelectric vibrator, and then
the silver electrode of the piezoelectric vibrator is bonded to the
cermet insulating material by the silver solder.
When a single crystal of the LN which is the trigonal system is
thermally expanded, anisotropy is present in a coefficient of
linear expansion, and even though the coefficient of linear
expansion in an X-axis direction and the coefficient of linear
expansion in a Y-axis direction are the same, the coefficient of
linear expansion in a Z-axis direction is different therefrom.
Considering that the metallic dumping portion is bonded to the
piezoelectric vibrator made of the LN single crystal, when the
dumping portion is bonded to a surface other than a surface (so
called "Z cut surface") orthogonal to the Z-axis in the LN, the
anisotropy occurs within the bonded surface during the thermal
expansion, and therefore cracks may be generated in the
piezoelectric vibrator due to a heat cycle applied thereto, and the
like. However, as described in Non-Patent Document 3 and the like,
a piezoelectric coefficient in the Z-axis direction in the LN
single crystal is smaller than that of other general piezoelectric
materials. For this reason, the ultrasonic sensor, in which the LN
piezoelectric vibrator is not damaged even if the heat cycle is
applied thereto, has reduced transmission or reception capabilities
of the ultrasonic wave and does not accurately measure the flow
rate. Table 1 shows characteristics such as the Curie point, the
piezoelectric coefficient, and a relative dielectric constant, in
various piezoelectric materials, and Table 2 shows a coefficient
thermal expansion (coefficient of linear expansion) in the LN or
other materials. In the Table 1, a Z cut plate represents an LN
plate cut along two parallel Z cut surfaces and a Y 36.degree. cut
plate represents the LN plate cut along two paralell Y-axis
36.degree. cut surfaces to be described below.
TABLE-US-00001 TABLE 1 LiNbO.sub.3 LiNbO.sub.3 Piezoelectric (Y
36.degree. cut (Z cut Material plate) plate) PbNb.sub.2O.sub.6
PbTiO.sub.3 PZT Curie Point (.degree. C.) 1150 1150 530 385 150 to
295 Piezoelectric 40 6 80 44 470 Coefficient D.sub.33 (pC/N)
Relative 39 29 300 185 1500 to Dielectric 3000 Constant .epsilon.
Density (g/cm.sup.3) 4.46 4.46 5.7 7.6 7.65 Sound 7340 3800 -- 4500
4600 Velocity (m/s)
TABLE-US-00002 TABLE 2 Coefficient of Linear Expansion at 25 to
Material 850.degree. C. (.times.10.sup.-6 K.sup.-1) LiNbO.sub.3
(X-axis direction, 5.15 to 2.25 Y-axis direction) LiNbO.sub.3
(Z-axis direction) 13.85 to 3.88 Silver 18.9 or more Stainless
Steel (SUS304) 14.8 Pure Titanium 8.4 or more Frit glass
(SiO.sub.2--B.sub.2O.sub.3--ZnO) 7.65
When the flowmeter is configured using the ultrasonic wave, for
example, it is necessary for the ultrasonic sensor to be
mechanically and acoustically bonded to the pipe or a spool piece
installed on the pipe. In this case, the ultrasonic wave from the
ultrasonic sensor needs to be efficiently transferred to the pipe,
the spool piece, or the like, and therefore a couplant (contact
medium) is applied to a contact portion of the pipe, the spool
piece, or the like. When the flowmeter for high temperature is
manufactured, as the couplant, a material withstanding high
temperature is used. For example, Patent Documents 6 to 8 disclose
a couplant which includes water glass as a main ingredient and has
appropriate flexibility or viscosity in a measurement temperature
region. After the ultrasonic sensor is manufactured, the couplant
including the water glass as a main ingredient is disposed to the
ultrasonic sensor by application, or the like. Patent Document 9
also discloses that an electrode made of a heat resistant soft
metal having appropriate plasticity in the measurement temperature
region and the electrode is used as the couplant. Further, as the
couplant for high temperature, examples using a gold foil or a
copper foil and silver have been known in the related art.
The ultrasonic flowmeter is used a principle of obtaining a flow
velocity based on, for example, the difference between the
ultrasonic transfer time in a flow direction and the ultrasonic
transfer time in a direction opposite to the flow direction, and
measuring the flow rate by the obtained flow velocity (Non-Patent
Document 1). Therefore, it is preferable that the ultrasonic
flowmeter has a small Q value for the ultrasonic signal and small
reverberation as a whole of the ultrasonic flowmeter. When the
zero-crossing method or the correlation measurement method is used
for measuring the transfer time difference, it is important to
reduce, in particular, the reverberation.
To reduce the reverberation, for example, a thin protective film or
a retarder is disposed on a front surface of an ultrasonic probe
for nondestructive inspection or medical treatment. The retarder
also serves as the above-described dumping portion. When an
acoustic impedance (herein, referred to as an "intrinsic acoustic
impedance" represented by a product of the sound velocity and the
density of the material) of the retarder is close to the acoustic
impedance of the piezoelectric vibrator, the ultrasonic wave
generated from the piezoelectric vibrator is transferred to the
retarder and vibration energy disappears by being scattered in the
vibrator. Therefore, multiple reflections, that is, resonances are
rapidly damped within the vibrator as much. As the acoustic
impedance of the vibrator is close to that of the retarder, the
vibration energy inside the piezoelectric vibrator is transferred
to an outside thereof, and therefore the Q value of the vibrator is
reduced and an output waveform thereof becomes a waveform having a
small ringing. However, a piezoelectric vibrator having a high Q
value has been used in the related art. Patent Document 3 discloses
that the reverberation appears to be small by using a propagation
auxiliary member in a two vibrator type ultrasonic probe used for
nondestructive inspection, or the like.
PRIOR ART DOCUMENT
Patent Document
(Patent Document 1) Japanese Patent Laid-Open Publication No.
2000-162004
(Patent Document 2) Japanese Patent No. 4205711
(Patent Document 3) Japanese Patent Laid-Open Publication No.
2006-090804
(Patent Document 4) Japanese Patent Laid-Open Publication No.
7-046095
(Patent Document 5) Japanese Patent Laid-Open Publication No.
10-339722
(Patent Document 6) Japanese Patent Laid-Open Publication No.
4-029056
(Patent Document 7) Japanese Patent No. 4244172
(Patent Document 8) Japanese Patent Laid-Open Publication No.
2005-064919
(Patent Document 9) Japanese Patent Laid-Open Publication No.
2008-256423
(Patent Document 10) Specification of U.S. Pat. No. 4,961,347
Non-Patent Document
(Non-Patent Document 1) "(revised version) Practical Navigation of
Flowmeter", association of weighing equipment industry society in
Japan which is general incorporated association, Kogyogijutsusha
Publication, pp 119-126 (September 2012)
(Non-Patent Document 2) R. Kazys, et al., "Research and development
of radiation resistant ultrasonic sensors for quasi-image forming
systems in a liquid lead-bismuth," ISSN 1392-2114 ULTRAGARSAS
(ULTRASOUND), Vol. 62, No. 3, pp. 7-15, 2007
(Non-Patent Document 3) Q. F. Zhou, et al., "Design and modeling of
inversion layer ultrasonic transducers using LiNbO.sub.3 single
crystal," Ultrasonics, Vol. 44, Supplement, pp. e607-e611, 2006
(Non-Patent Document 4) Ikeda Dakuro, "Basis of Piezoelectric
Material", OHM Co. 1984
(Non-Patent Document 5) K. K. Wong edit, "Properties of Lithium
Niobate," EMIS datareviews series No. 28, INSPEC, 2002
DISCLOSURE
Technical Problem
When configuring an ultrasonic sensor available in a high
temperature region using the piezoelectric vibrator made of lithium
niobate (LN), in order to prevent the vibrator from being damaged
in consideration of the coefficient of thermal expansion and the
anisotropy of the LN, a piezoelectric vibrator, which has a surface
(Z cut surface) orthogonal to the Z-axis of the crystal as an
output surface of the ultrasonic wave, has been used the related
art. However, since the piezoelectric constant in the Z-axis
direction in the LN is small, the ultrasonic wave may not be
generated at a high output, and thereby it is not possible to
detect the ultrasonic wave with high sensitivity.
As an orientation having a large piezoelectric constant in the LN,
an orientation in which the Y-axis of the crystal rotates to the
vicinity of the X-axis by about +36.degree. (for example,
36.degree..+-.2.degree.) has been known in the related art (for
example, Non-Patent Document 3). In order to obtain an ultrasonic
sensor having a high ultrasonic wave output and high sensitivity of
ultrasonic wave, an ultrasonic sensor, which is configured in such
a manner that a surface (hereinafter referred to as a "Y-axis
36.degree. cut surface") orthogonal to a direction in which the
Y-axis rotates to the vicinity of the X-axis by about +36.degree.
is formed as the output surface of the ultrasonic wave, may be
considered. FIG. 1(b) describes the Y-axis 36.degree. cut surface
of the LN and illustrates that the Y-axis 36.degree. cut surface
may be obtained by rotating the surface (Y cut surface) orthogonal
to the Y-axis to the vicinity of the X-axis by +36.degree.. FIG.
1(b) also illustrates the Z cut surface for reference. However,
since the coefficient of linear expansion is different in two
in-plane directions in a surface other than the Z cut surface, that
is, a surface which is set an inclined direction in the Z-axis
direction as a normal surface, a uniform coefficient of linear
expansion may be obtained and when for example, the retarder or the
dumping member is bonded to the surface, cracks may be generated in
the crystal. Therefore, there is a need to form the Y-axis
36.degree. cut surface as the output surface and develop a bonding
method for preventing cracks from being generated in the crystal
when the retarder, and the like is boned thereto. A detailed value
for the coefficient of linear expansion at the Y-axis 36.degree.
cut surface may not be found, but is considered to be an
intermediate value between the coefficient of linear expansion in
the Z-axis direction and the coefficient of linear expansion in the
X-axis direction (the coefficients of linear expansion in the
X-axis direction and the Y-axis direction are the same). In
consideration of an inclined angle from the Z-axis, the value may
be estimated as about 7 to 10.times.10.sup.-6 K.sup.-1.
When the Y-axis 36.degree. cut surface is formed as the output
surface, the crystal is bonded to the retarder by a bonding
material to prevent cracks from being generated in the crystal, and
therefore in a range of temperature applied during the bonding or a
use temperature range of the ultrasonic sensor, it is necessary for
the crystal, the retarder, and the bonding material to have a value
of the coefficients of linear expansion approximate each other.
Further, in the above-described conventional ultrasonic sensor, a
couplant (contact medium) needs to be disposed between the pipe or
the spool piece to which the sensor is attached and the output
surface of the ultrasonic wave (for example, an emission end face
of the ultrasonic wave) to improve mechanical coupling in terms of
the ultrasonic wave. In a conventional ultrasonic sensor for high
temperature, it is difficult to rapidly dispose the ultrasonic
sensor in the pipe or the spool piece which is an object to be
inspected by additionally applying or attaching couplant materials
such as a gold foil, a copper foil, an aluminum foil, a polyimide
foil, and water glass to the ultrasonic sensor later. Therefore, a
method for forming a soft metal serving as the couplant on the
output surface of the ultrasonic wave during manufacturing the
ultrasonic sensor is required.
It is necessary for the ultrasonic sensor to improve measurement
precision by using a single crystal which may suitably separate a
longitudinal wave and a transverse wave from each other to
propagate a high-quality signal. It has been known that it is
effective to bond the titanium retarder to the piezoelectric
vibrator, but if the bar-shaped titanium retarder is bonded to a
single crystal LN, the ultrasonic wave from an end face at a side
opposite to the bonded surface in the retarder is multi-reflected,
and thus a phase thereof is inverted 180.degree.. For this reason,
the reflection wave thereof as reverberation overlaps an original
ultrasonic transfer signal to cause deterioration in signal
processing for measurement. In particular, when using the
zero-crossing method or the correlation method to measure the
transfer time difference, if the multiple reflection waves overlap
the original received signal under any circumstances, it is not
possible to accurate measure the flow rate.
Accordingly, in consideration of the above-described circumstances,
it is an object of the present invention to provide an ultrasonic
sensor which includes a piezoelectric vibration generating a high
ultrasonic wave output by having a Y-axis 36.degree. cut surface of
LN as an output surface and may be used in a high temperature
region and prevent cracks from being generated in a crystal, and a
manufacturing method thereof.
Technical Solution
According to one aspect of the present invention, there is provided
an ultrasonic sensor, including: a piezoelectric vibrator which is
made of lithium niobate and has a Y-axis 36.degree. cut surface as
an output surface; a retarder made of titanium; and a bonding layer
for bonding one surface of the retarder to the output surface,
wherein the bonding layer is made of silver and frit glass, and the
frit glass has a coefficient of linear expansion ranging from
5.times.10.sup.-6 K.sup.-1 to 15.times.10.sup.-6 K.sup.-1.
According to another aspect of the present invention, there is
provided a manufacturing method of an ultrasonic sensor having
lithium niobate as a piezoelectric vibrator, including: forming the
piezoelectric vibrator which has a Y-axis 36.degree. cut surface as
an output surface, and is prepared by applying a silver paste to at
least the output surface and burning the same; applying the silver
paste to one surface and the other surface of a titanium retarder
which are opposite to each other; and contacting one surface of the
retarder with the output surface of the burned piezoelectric
vibrator, and then performing burning the same under an inert gas
atmosphere at a predetermined temperature or more, wherein the
silver paste applied to the output surface and the one surface of
the retarder includes silver and frit glass, and the frit glass has
a coefficient of linear expansion ranging from 5.times.10.sup.-6
K.sup.-1 to 15.times.10.sup.-6 K.sup.-1.
In the present invention, the Y-axis 36.degree. cut surface is
referred to a surface obtained by rotating a surface orthogonal to
the Y-axis of the LN crystal about the X-axis by about +36.degree.
(for example, 36.degree..+-.2.degree.).
Further, an example of the inert gas may include nitrogen and argon
and the predetermined temperature may be, for example, 500.degree.
C.
Advantageous Effects
According to the present invention, by using the silver and frit
glass materials for bonding the LN piezoelectric vibrator to the
titanium retarder, the Y-axis 36.degree. cut surface capable of
providing high output may be used as the output surface of the LN
piezoelectric vibrator without damaging the piezoelectric
vibration, and the like even when used in the high temperature
conditions.
DESCRIPTION OF DRAWINGS
FIG. 1(a) is a diagram illustrating a crystal structure of lithium
niobate (LN) and (b) is a diagram for describing a Y-axis
36.degree. cut surface of LN.
FIGS. 2(a) and (b) are a top view and a side view of an ultrasonic
sensor which is one embodiment of the present invention,
respectively.
FIG. 3 is a flow chart illustrating a manufacturing process of the
ultrasonic sensor illustrated in FIG. 2.
FIG. 4 is a diagram illustrating a transmitting and receiving
configuration for measuring a flow rate.
FIG. 5 is a waveform diagram illustrating a burst waveform for
driving the ultrasonic sensor.
FIG. 6 is a schematic cross-sectional view for describing multiple
reflections in the ultrasonic sensor.
FIGS. 7(a) and (b) are graphs for describing an angle of beam
spread.
FIGS. 8(a) and (b) are graphs for describing a multiple reflection
waveform.
BEST MODE
Hereinafter, embodiments of the present invention will be described
with reference to the accompanying drawings. FIG. 2 illustrates an
ultrasonic sensor which is one embodiment of the present
invention.
The ultrasonic sensor includes a single crystal of lithium niobate
(LN: LiNbO.sub.3) as a piezoelectric vibrator 1. The piezoelectric
vibrator 1 has a disk shape. In the piezoelectric vibrator 1, both
of a bottom surface and a top surface which are formed in a disk
are a Y-axis 36.degree. cut surface of LN. In addition, one surface
(bottom surface in FIG. 2) of two surfaces (the above-described
bottom and top surfaces) of the piezoelectric vibrator 1 which face
each other is an output surface through which an ultrasonic wave is
output from the piezoelectric vibrator 1. The output surface is
bonded to one end of a round bar-shaped titanium retarder 3 through
a bonding layer 2. The titanium retarder 3 is formed of pure
titanium (Ti). First of all, the titanium retarder 3 may inevitably
include impurities contained in titanium. The bonding layer 2 is
formed by burning a silver paste including frit glass as described
below. Therefore, the bonding layer 2 is made of silver and frit
glass. Herein, the frit glass having a coefficient of linear
expansion ranging from 5.times.10.sup.-6 K.sup.-1 to
15.times.10.sup.-6 K.sup.-1 is used. The other end of the titanium
retarder 3 is provided with a couplant 4 including silver. In order
to form the couplant of the ultrasonic sensor simultaneously with
manufacturing a main body of the sensor without subsequently adding
the couplant, it is preferable that the couplant 4 having the same
composition as that of the bonding layer 3 is simultaneously formed
with the bonding layer 3.
FIG. 3 illustrates an example of a manufacturing process of the
ultrasonic sensor.
For example, opposite surfaces of the LN single crystal
(piezoelectric vibrator 1) having a thickness of 0.8 mm to 1.6 mm
and a diameter of 10 mm to 18 mm are applied with the silver paste
for an electrode and are burned at 700 to 850.degree. C. (step 11).
Further, both end faces of a round bar (diameter of 20 mm and
length of 20 mm) of the titanium retarder 3 made of a pure titanium
material are applied with the silver paste and dried at 80.degree.
C. (step 12). Further, contacts a tip portion of the titanium
retarder 3 with the LN single crystal (piezoelectric vibrator 1)
having the burned electrode (step 13), and performs burning the
same as a whole (step 14). For the burning, an inert atmospheric
burning furnace is used and the LN bonded with the titanium
retarder is stored and maintained in the air for a period until a
temperature in the burning furnace reaches 500.degree. C., that is,
for 3 hours to evaporate a binder ingredient included in the silver
paste. Next, the temperature in the burning furnace rises from
500.degree. C. to a range of 700 to 850.degree. C. for 2 hours
under the inert gas or the air atmosphere. The reason of
maintaining the burning furnace in the inert gas or the air
atmosphere is to prevent a titanium oxide layer from being formed
in the bonding layer 2 or control the oxide layer. As the inert
gas, for example, argon (Ar), nitrogen (N.sub.2), a mixed gas
thereof may be used. The LN bonded with the titanium retarder is
stored and maintained at 700 to 850.degree. C. for 30 minutes and
then is cooled to a room temperature for 10 hours under the inert
gas or the air atmosphere (step 15). The ultrasonic sensor
according to the present embodiment is completed by the
above-described process.
According to the present invention, the titanium retarder 3 is
bonded to the Y-axis 36.degree. cut surface of LN. Herein, in order
to prevent a damage in the piezoelectric vibrator (LN crystal) 1,
the bonding using a silver paste containing frit glass is
performed. In this case, the silver paste used herein includes 79
to 82% of silver and 2.3 to 2.5% of frit glass ingredient, in terms
of a mass ratio, and an organic binder ingredient as a balance. The
organic binder ingredient mainly includes diethyleneglycol mono
n-butyl ether or ethyl cellulose.
According to the present embodiment, kinds of frit glass mixed with
the silver paste may include, for example,
SiO.sub.2--ZnO--B.sub.2O.sub.3 (zinc borosilicates) and
B.sub.2O.sub.3--ZnO--Al.sub.2O.sub.3 (zinc alumina borates), and
materials having a coefficient of linear expansion ranging from
5.times.10.sup.-6 K.sup.-1 to 15.times.10.sup.-6 K.sup.-1,
preferably 7 to 8.times.10.sup.-6 K.sup.-1 may be used. In this
case, a composition of the silver paste may include 82% of Ag, 0.1%
of Si, 0.05% of Al, 0.2% of B, and 1.0% of Zn, in terms of mass %.
Further, the frit glass may be
(Al.sub.2O.sub.3--B.sub.2O.sub.3--ZnO--CoO--K.sub.2O--CaO--SnO)--SiO.sub.-
2 based frit glass and materials having a coefficient of linear
expansion of 7.6.times.10.sup.-6 K.sup.-1 may be used. In this
case, a composition of the silver paste may include 81% of Ag, 0.4%
of Si, 0.2% of Al, 0.2% of B, 0.01% of Zn, and 0.02% of Co, in
terms of mass %. According to the reviews of the present inventors,
the frit glass including cobalt is preferably used. As can be seen
from the above Table 2, the coefficient of linear expansion of the
glass ingredient is 7.65.times.10.sup.-6 K.sup.-1, which is close
to a coefficient of linear expansion of titanium material of
8.4.times.10.sup.-6 K.sup.-1. For the coefficient of linear
expansion after the silver paste is burned, the coefficient of
linear expansion of glass having 7.65.times.10.sup.-6 K.sup.-1 is
dominant compared to the coefficient of linear expansion of silver
having 18.9.times.10.sup.-6 K.sup.-1. For this reason, the
ultrasonic sensor according to the present embodiment has in-plane
anisotropy in the coefficient of linear expansion at the Y-axis
36.degree. cut surface of LN to prevent the bonded portion from
delaminating or cracks from being generated in the crystal.
According to the present embodiment, the bonding layer 2 has a
thickness of, for example, 10 .mu.m to 30 .mu.m and the couplant 4
has a layer thickness of 5 .mu.m to 20 .mu.m.
Hereinafter, the material of the retarder will be described. The
ultrasonic sensor according to the present embodiment is
manufactured by the above-described process. Herein, in order to
reduce thermal oxidation in a temperature range of 500 to
850.degree. C. subjected during bonding the retarder to the
piezoelectric vibrator and in the viewpoint of processing property,
pure titanium, not stainless steel, is used. For the coefficient of
linear expansion, the pure titanium has a coefficient of linear
expansion having 8.4.times.10.sup.-6 K.sup.-1 closed to a
coefficient of linear expansion having 7 to 10.times.10.sup.-6
K.sup.-1 which is considered at the Y-axis 36.degree. cut surface
of LN. According to the reviews of the present inventors, when the
retarder is made of a titanium alloy, a difference between the
coefficients of linear expansion of the piezoelectric vibrator and
the retarder is increased and therefore it is believed that the
delaminating of the bonded portion, the damage in the crystal, or
the like may easily occur.
FIG. 4 illustrates a configuration for measuring a flow rate using
the ultrasonic sensor according to the present embodiment. Herein,
hot water flowing in a pipe 21 is an object to be measured the flow
rate. An upstream side and a downstream side of the pipe 21 are
mounted with spool piece parts 22, respectively. Each spool piece
part 22 has a spool piece 23 which is made of cast iron and
configured to transfer an ultrasonic wave to water in the pipe 21.
The ultrasonic sensor 20 according to the present embodiment is
bonded to the spool piece 23 from the outside so that the spool
piece 23 is pressure-welded to the titanium retarder 3 through the
couplant 4. Herein, a temperature of the water in the pipe 21 is
set to be, for example, 230.degree. C. which is much higher than
the boiling point (100.degree. C.) of water under the atmospheric
pressure. Since this temperature is closed to a critical point of
water, a pressure of the water in the pipe 21 is, for example, 20
MPa.
In the configuration illustrated in FIG. 4, the spool pieces 23
made of cast iron are mounted inside each spool piece part 22 and
the ultrasonic sensor 20 is mounted in the spool piece part 22 from
the outside, and therefore the ultrasonic sensor 20 does not
contact water. However, since the ultrasonic sensor 20 is subjected
to heat from the spool piece part 22, it needs to be operated at a
high temperature.
Herein, the ultrasonic sensor 20 is driven by a burst wave of 4 MHz
which is formed of 5 voltage peaks (that is, 5 waves) as
illustrated in FIG. 5, and includes a signal generator 25 for
generating the burst wave. The burst wave generated from the signal
generator 25 is amplified to a sine wave of 100V.sub.p-p by an
amplifier 26 and is transferred to a (high frequency) RF switch 27.
The RF switch 27 functions to change transmission and reception
between the ultrasonic sensor 20 of the upstream side and the
ultrasonic sensor 20 of the downstream side at every 1 ms. When the
sensor of the upstream side transmits an ultrasonic burst signal
and the sensor of the downstream side receives the burst signal for
1 ms (T1), the sensor of the upstream side receives the ultrasonic
burst signal and the sensor of the downstream side transmits the
ultrasonic burst signal for subsequent 1 ms (T2). The RF switch 27
outputs the signal received by the ultrasonic sensor at a receiving
side, in which the signal is recorded by a 50-ohm terminated
digital oscilloscope 28. The digital oscilloscope 28 receives a
trigger signal from the signal generator 25. The transfer time
difference may be obtained from data recorded in the digital
oscilloscope 28, and therefore a flow velocity and a flow rate may
be calculated based on a shape of the pipe 21, a disposition of
each ultrasonic sensor 20, acoustic nature of the object to be
measured (herein, water) from the transfer time difference.
Next, the multiple reflections from the end face of the retarder
will be described. FIG. 6 describes a principle of the generation
of the multiple reflections.
In the end face (end face of the couplant) of a side opposite to a
bonded surface between the titanium retarder 3 and the
piezoelectric vibrator 1, acoustic impedance is different between
ultrasonic emitting line sides, and therefore in the end face, the
ultrasonic wave is reflected and at that time, a phase shift of
180.degree. occurs. The reflected ultrasonic wave is propagated to
the bonded surface side and is reflected from the bonded surface to
be again propagated to the end face side while leading to the phase
shift of 180.degree.. For this reason, propagation waveforms are
repeatedly generated.
A sound velocity in titanium may be changed due to shape factors
such as a diameter, but a longitudinal wave is basically
represented by (modulus of elasticity/density).sup.1/2, and
therefore by substituting this value in the titanium, (modulus of
elasticity/density).sup.1/2=(116/4.056).sup.1/2 [m/s]=5348 m/s.
Since the reflection wave is a plane wave going straight up to a
Fresnel zone limit, in the case of obtaining a value L of the
Fresnel zone limit, L=(radius).sup.2/4.lamda. when a wavelength is
defined as .lamda., and therefore when a radius of the titanium
retarder is 10 mm at a frequency of 4 MHz,
L=(10.sup.-2).sup.2/(4.times.5348/(4.times.10.sup.6))=19 mm, and
when the radius of the titanium retarder is likewise 10 mm at a
frequency of 2 MHz, L=10 mm. The length of the retarder is longer
than the Fresnel zone limit. In order to increase the length of the
retarder, there is a need to use a material having a low sound
velocity. Meanwhile, the titanium material has a lower sound
velocity than other materials and therefore is advantageous.
Further, in Equation representing the Fresnel zone limit, the
larger the radius, the longer the length of the retarder. However,
the angle (angle at which a sound pressure is 50%) of beam spread
is increased according to increase of the radius of the retarder,
and thus the precision of the measured flow rate is reduced.
Thereby, in the viewpoint of the accurate flow rate measurement,
there is a limitation in increasing the length of the retarder.
For the calculation of the angle of beam spread, an intensity of
central sound field may be represented by an approximate equation
of a squared, rectangular sound field having a=2 cm (corresponding
to a radius of 10 mm), not a circular sound field requiring a
Bessel function of the first kind. The approximate equation is
represented as follows.
.function..times..times..times..times..theta..times..times..times..times.-
.theta..times..times..times..pi..lamda..times..times..times..times.
##EQU00001##
FIG. 7 illustrates a result of an approximate calculation in the
rectangular sound field, in which (a) illustrates a frequency of 2
MHz, and (b) illustrates a frequency of 4 MHz. From the result, it
may be appreciated that the angle of beam spread at 4 MHz is
0.63.degree. and the angle of beam spread at 2 MHz is
1.37.degree..
The reflection at the boundary may be calculated from an
(intrinsic) acoustic impedance of materials disposed at both sides
of the boundary by a calculation equation of the Fresnel
reflection. Table 3 shows the density, sound velocity, acoustic
impedance of each material.
TABLE-US-00003 TABLE 3 LiNbO.sub.3 (Y36.degree. cut plate) Titanium
Iron Cast Iron Silver Water Sound velocity c (m/s) 7340 5348 5500
4910 3650 1480 Density .rho. (.times.10.sup.3 kg/m.sup.3) 4.46 4.51
7.86 7.86 10.5 1 Acoustic Impedance (kg/m.sup.2 s) 32.7 18.67 43
38.5 38.32 1.48
The reflection at the boundary from the titanium retarder to the
silver couplant is calculated by the following Mathematical
Equation 2.
.times..times..times..times..times. ##EQU00002##
.rho..times..rho..times..rho..times..rho..times. ##EQU00002.2##
Similarly, the reflection at the boundary between the bonding layer
made of silver and the LN single crystal is calculated by the
following Mathematical Equation 3.
.times..times..times..times..times. ##EQU00003##
.rho..times..rho..times..rho..times..rho..times. ##EQU00003.2##
Meanwhile, the reflection at the boundary from the silver couplant
to the cast iron is represented by the following Mathematical
Equation 4 and since the acoustic impedances thereof approximate
each other,
.times..times..times..times. ##EQU00004## the reflection is a small
value which may be disregarded.
The reflection when the ultrasonic wave is output from the spool
piece of cast iron to water is represented by the following
Mathematical Equation 5, and
.times..times..times..times. ##EQU00005## 15% of a non-reflection
side to a total reflection wave is output to water.
If a wave which is reflected from the boundary between the titanium
retarder 3 and the couplant 4 made of silver, and is again
reflected from the boundary with the piezoelectric vibrator is
referred to the reflection wave, it is necessary for the titanium
retarder 3 to have a length which does not occur a multiple
reflection in which the reflection wave overlaps the measurement
wave. FIG. 8(a) illustrates an example in which there is a
sufficient time interval, and FIG. 8(b) illustrates an example in
which there is no influence of the multiple reflections, but the
reflection overlaps the waveform as reverberation due to the
influence of the multiple reflections. For example, when the length
of the titanium retarder 3 is set to be 10 mm, the frequency of the
burst signal is set to be 4 MHz (time per one wave is 0.25 .mu.s),
the number of waves in the signal is set to be 5 waves, and the
piezoelectric vibrator is driven at the 5 waves, and then is dumped
to sink vibration at the 5 waves, as the length of the titanium
retarder 3, a time corresponding to 10 waves which are two times as
large as a driving waveform is at least required. This becomes
0.25.times.10=2.5 .mu.s. When the length of the titanium retarder 3
is set to be 10 mm, a reciprocating propagation time of the
ultrasonic wave at the retarder is
2.times.10.times.10.sup.-3/5348=3.7 .mu.s, and an interval with the
multiple reflection wave is 12 .mu.s. Meanwhile, when the frequency
is 2 MHz, at least required time of the 10 waves is about
0.5.times.10=5 .mu.s, and the multiple reflection wave overlaps the
signal for measurement. When the length of the retarder is L, the
frequency is f, the number of waves in the burst wave is N, and the
sound velocity in the retarder is v, to prevent the influence of
the multiple reflection wave, the driving time and the dumping time
of the piezoelectric vibrator are the same, The relationship of
(L/v)>(N/f) needs to be satisfied.
As described above, the ultrasonic sensor according to the present
embodiment using the LN piezoelectric vibrator by using the silver
and frit glass materials for bonding with the titanium material
forming the retarder may use the Y-axis 36.degree. cut surface
which may implement the high output as the output surface of the
piezoelectric vibrator, while preventing the piezoelectric vibrator
from being damaged or the bonded portion from being delaminated.
Further, the ultrasonic sensor uses the silver and frit glass
materials used for bonding the titanium retarder as the couplant
material to the piezoelectric vibrator, and as a result, there is
no need to additionally dispose the couplant material later when
the ultrasonic sensor is attached to the pipe or the spool piece.
Further, in the titanium retarder, the dimension conditions which
are not affected by the multiple reflection makes certain, and thus
the high-quality signal may be processed.
DESCRIPTION OF REFERENCE NUMERALS
1: piezoelectric vibrator, 2: bonding layer
3: titanium retarder, 4: couplant
20: ultrasonic sensor, 21: pipe
22: spool piece part, 23: spool piece
* * * * *